Abrasive and method of making same



' April 25, 1944. c; BENNER ETAL 2,347,537

ABRASIVE AND METHOD OFMAKINGSAME Original Filed June 12, 1941 than plane surfaces.

Patented Apr. 25, 1944 UNITED STATE ABRASIVE AND METHOD OF MAKING SAME Raymond C. Bonner and Frederick A. Upper, Ni-

agara Falls, N.

Y., assignor to The Carborundum Company, Niagara Falls, N. Y., a corporation of Delaware Original application June 12, 1941, Serial No.

397,776. Divided and this application July 29, 1942, Serial No. 452,730

6 Claims. (Cl. 51-309) This invention relates to a new and improved abrasive material for finish grinding and polishing and to the method of making the same. Abrasive articles made from such new and improved abrasive material are suitable for performing finish grinding and polishing operations, as for example abrasive wheels or runners for the surfacing of plate glass sheets and the like. More particularly, the present abrasive material and the abrasive portion of articles made therefrom are constituted of fine abrasive particles of alpha alumina in the form of essentially complete individual crystals or primary fragments of crystals which have plane or fiat surfaces and sharp angular crystal corners and edges. The product of the present invention is particularly effective for the surfacing of plate glass sheets and like materials and therefore will be described more specifically as it pertains thereto.

This application is a division of application Serial No. 397,776, filed June 12, 1941, and entitled Abrasive products and method of making and using the same.

Ordinary (artificial) abrasive particles, such as fused alumina (A1203) for example, are obtained by crushing a relatively large mass which has been produced by the well known present commercial method of fusion in an electric furnace. The mass formed by the relatively slow cooling of the fused material is anhedral, that is, it is substantially lacking in individual crystals. Natural crystal formation is inhibited in all directions by the presence of adjacent material. As a result this mass of alumina when solidified is composed of intergrown and contiguous grains entirely without crystal form. It is the habit of this material when crushed to smaller size to break with a conchoidal fracture, that is, to break in random directions with the formation of curved rather Some edges and corners are formed at the intersection of these curved surfaces. However, the continued reduction in particle size necessary to obtain the extremely finely divided material required for finish grinding or polishing tends to destroy more and more of these edges and corners and to accentuate the curved surfaces to such a degree that the final product is rounded rather than angular.

The present invention provides an abrasive material for polishing and similar surfacing operations which has improved finishing or polishing actions and is more eficient in operation than is that produced in the manner described above.

In order to more clearly understand the nature of the invention, reference is made to the drawing, in which Figure 1 shows the finely divined abrasive polishing material of the present invention as seen at a magnification of 500 diameters under the microscope; and

Figure 2 shows a similarly magnified lot of ordinary fused crushed alumina polishing material made acrording to prior art methods.

Figure 3 shows a top plan view of an inverted cup-shaped surfacing disc having a frusto-conical grinding surface.

Figure 4 shows a vertical diametrical crosssection through the line IV-IV of Figure 3.

The material shown in Figure 1 is a finely comminuted alumina polishing material made .according to the present invention by formation of alpha alumina bubbles and gently crushing them as hereinafter described. The material shown is of an order of fineness of 600 mesh to the linear inch and finer. The individual particles substantially all are 15 microns or smaller diameters. The individual particles have been formed by fracture of the original'bubbles along the boundary lines of individual crystals, thereby producing a relatively large number of complete, rhombohedral crystals 3 of alpha alumina. Other sharp pyramidal primary fragments t have been produced by primary cleavage of various crystalline rhombohedra along lines parallel to the base. As a result the finely divided material consists largely of complete individual crystals 3 and primary pyramidal fragments t of crystals. Both the full rhombohedra and the fragments'have sharp straight cutting edges and angular corners.

Figure 2 showing a similarly magnified lot of ordinary fused crushed alumina is offered for purposes of comparison and to more clearly distinguish the present material from that of the prior art. The material of Figure 2 is also at 500 diameters magnification and 600 mesh and finer in size; in other words, of the same order of fineness as the material shown in Figure 1. However, 4 it has been made by prior art methods by crushing an ordinary mass of fused alumina to the required fineness. The material contains no complete crystals and it is constituted for the most part of rounded pieces b resulting from repeated fracturing of large crystal masses and showing no resemblance to any crystal form. Occasionally such material contains remnants t of several crystal forms clustered, but otherwise the corners of the particles are rounded off and the ed es curved as a result of the random conchoidal fracturing over and over again of the original fused mass.

The surfacing disc shown in Figure 3 and 4 is of an inverted cup shape having a base portion I and a circumferential ring or rim portion 8. The

- disc shown is provided with a centrally located arbor 9 with bushing l suitable for mountin upon a suitable shaft (not shown). The grinding surface I i is frusto-conical, and in operation the shaft axis is inclined slightly from normal to the work being surfaced so that the grinding surface I l is presented to the work along a line contact perpendicular to the circumference of the wheel. The slope of the frusto-conical grinding surface H is such that the angle of tilt of the rotating shaft from the perpendicular is small, usually being only about 1 or 2. By this grinding.with a frusto-conical surface, the grinding is comparable to that of peripheral grinding with a wheel of abnormally large diameter. The depth and width of the rim portion 8 may vary considerably in practice, depending upon the use to which the wheel or disc is to be put. Also, the back or base portion 1 can be reinforced, if desired, with a metal or other rigid backing plate.

In accordance with the present invention aluminous material is fused and then cooled from the molten state while freely suspended in droplet form by one of the known methods such as subjecting a thin stream of the molten A120: to a blast of air under pressure to atomize the molten alumina. The solidification of the molten material in this manner results in the formation of small separate shot-like hollow globules or bubbles.- Each bubble so formed consists of a great number of completely developed, minute crystals of the habit common to that material. In the case of A1203 these crystals are found to be first order positive rhombohedrons.

It is pointed out at this time that it is not desired to claim the manufacture of A1203 bubbles as novel. Such bubbles have been previously used in making refractory insulating bricks and in the production of aluminum metal.

It is significant to the practice of the present invention that cooling as carried out in the formation of the hollow bubbles above in a large measure controls crystal size and form and is an important step in the process of producing the material of the present invention. In typical air blown bubbles, the characteristic individual crystals of alpha alumina are chiefly of the size of microns and smaller. Cohesion between the many crystals that form the hollow bubble is relatively weak, so that they can be separated from each other by a gentle crushing action which is not severe enough to fracture the individual crystals in most cases. Should some of these -minute crystals fracture during crushing, or

should it be desirable to crush them to a size somewhat smaller than the crystal size, the

natural crystal edges and comers will be retained by the fragments.

Crushing by a gentle mulling action, such as is afforded in a slowly revolving ball mill with a charge of small balls' or by other means whereby strong ,splintering blows are avoided, has been found extremely satisfactory in promoting preferential shattering or fracturing of the hollow bubbles between the crystals instead of across the crystals, and when crystals are broken by such mulling, the tendency is for them to break along primary lines of cleavage to produce the sharp pyramidal primary fragments shown in the drawing. The size of the crystals is so small as to require only one or two fractures of even the largest crystals to reduce them to the required fineness.

Having thus obtained abrasive particles that are essentially all individual, complete crystals of the desired size, or primary fragments thereof, as above described, they are formed into an abrasive article by any of the methods well known in the abrasive art. That is to say, a bonding material is intimately and uniformly admixed with the particles of crystal A1203, the mixture compacted and formed to the desired shape and then subjected to a treatment suitable to the curing or maturing of the bonding agent. Although any abrasive bonding material may be used, including ceramic, silicate, rubber, resin, .1 other common bonding materials, organic and particularly resin bonds of the phenolformaldehyde and glycerine-phthalic acid types have been found to produce very satisfactory articles. The abrasive particles so produced may also be advantageously used as loose, unbonded polishing grain in the manner familiar with other abrasive materials applied in conjunction with polishing pads or runners.

To accomplish the purpose of this invention and to meet the variance in conditions imposed in the practice of it the following typical examples and methods of molding polishing wheels employing as the abrasive therein material made according to the present invention are given. It is to be understood, however, that the examples are illustrative only and not restrictive.

EXAMPLE I 9.30 grams of crushed alpha alumina bubbles produced in the manner herein described above, of such fineness that 80% of the material will be of the size equivalent to a 600 mesh screen are distribution of the resin through. the abrasive powder. After thorough mixing, the mixture is then put in a suitable mold, such as is commonly used for forming abrasive articles, and pressed under high pressure, for example, 2000 pounds per square inch. While under pressure the plunger of the mold may be clamped in place by any convenient means and the clamped mold removed from the press and placed in an oven, the temperature of which is about 350 F. The molded article is cured at this temperature for a period of from twelve to sixteen hours, after which it may be removed from the mold and flnished by machining in the manner heretofore .used in finishing abrasive articles.

By a modification of this method the article may be pressed at the high pressure, e. 'g.,' 2000 pounds per square inch, removed from the press and cured in an oven under a pressure of the or- 7-6 under reduced pressure to soften the resin grains slightly, cooled, removed from the mold and cured by suitable means.

EXAMPLE II Parts by weight 600 mesh and finer crushed alumina bubbles--. 7 5 Rubber l Sulphur Filler The above mixture for making rubber-bonded polishing wheels is processed in the following manner. The crude rubber is first plasticized to the desired degree by milling on rolls or in a Banbury mixer. The compounding ingredients, such as sulphur and filler, are next added with continued milling until a homogeneous mass is formed. The filler may consist of whiting, magnesia, clay, cryolite-and similar reinforcing or modifying materials well known to the art.

After all the other ingredients have been thoroughly incorporated in the rubber, a portion of the abrasive particles is placed between a fold of the unvulcanized compounded rubber and passed through the rolls. This operation is repeated until the entire amount of grain has been incorporated uniformly throughout the rubber mass. The abrasive-rubber mix is then sheeted to the required thickness and pieces of the desired shape died or cut out of the sheet and placed in a steel mold for curing. The abrasive article is matured by curing the mass in the mold at low pressure at 287 F. for 2 hours, followed by an open steam cure for 10 hours at the same temperature.

EXAMPLE 111 Parts by weight 600 mesh and finer crushed alumina bubbles 82 Ceramic bond 18 Cellulose sulfite pitch (dry) 5 The above dry ingredients are mixed thoroughly and sufficient water added to moisten Frit composition Parts by weigh Borax 21.5 Soda ash 21.5 Flint 57.0

One form of wheel that has been found to be very satisfactory for carrying out the surfacing operation on plate glass known as smoothing is one in the shape of a cup which is held in an inverted position, by suitable well known means, and is caused to rotate with the rim of the cup in contact with the surface to be ground. By tilting the spindle that drives the wheel slightly from the normal to the surface the nature of the contact between the wheel and the work is modified in a very important manner. When the axis of such a wheel is tilted in this manner the grinding face of the wheel (since the wheel is substantially self-dressing) assumes the shape of a frustum of a right circular cone with a large vertex angle. While the area of contact between the tilted wheel and surface is spoken of as a would be produced by an ordinary,plain wheelmany, times the diameter of the cup wheel if grinding on its peripheral surface. For example, a cup shaped wheel having an outside diameter of 20" and an inside diameter of 14'', if mounted on a spindle, as above described and tilted at an angle of 1 /2" to the perpendicular to the surface, generates an arc of contact equivalent to that of a plain wheel approximately 650" in diameter grinding on the periphery. When a wheel employing the abrasive material of the present invention thus tilted is applied to glass surfaces, a distinctive type or quality of surface is produced. It is more transparent than any directly ground surface generated by any other present known means and is virtually free from pits. It can be further described as being in the form of extremely minute striations rather than of the chipped and pitted, scoured surfaces commonly produced by grinding with loose abrasives.

in producing polished glass surfaces of the present invention, abrasive may be applied at any stage prior to the rouge polishing operation, but is perhaps most effectively applied to glass which has been given a preliminary grinding operation with sand beneath iron runners in the manner customary in surfacing plate glass. This is the smoothing stage at which emery is frequently used, but our material gives higher transparency of the glass than does emery, particularly when our material is used in the form of frusto-conical wheels which may be mounted to sweep the surface of an underlying train of glass sheets as it passes beneath the wheels. Such wheels are rotated about their own axes and may also revolve in a planetary orbit about an axis which may be common to two or more such wheels. The material and/or wheels containing it maybe used either alone .or as a step in the process of polishing in conjunction .with other materials used in other runners or laps.

It has been found that the present nove abrasive article when substituted for the conventional smoothing" operation produces a smooth characterized by fewer and shallower pits and holes, and that this improved surface is obtained with appreciably lower expenditure of time and power. A further manifestation 0i this improved surface is evidenced by the fact that only about half the usual amount of final polishing isv required.

It has been found that in grinding of glass surfaces with the abrasive produced by the present invention asatisfactory surface superior to that now produced commercially, as will be explained subsequently in greater detail, is generated by maintaining the axis of wheel rotation perpendicular to that surface. In such case the area of contact between wheel and glam surface is equal to the grinding face area of the wheel.

An abrasive article formed of the materials and in the manner herein described is particularly well adapted to the finish grinding of relatively hard, brittle materials such as ceramic ware, stone and glass.

While the abrasive polishing material made in accordance with the present invention has been described as it is used in bonded form for performing finish grinding and cutting actions, such as the smoothing of plate glass sheets, its utility is not limited thereto. The present sharp, fast cutting crystalline polishing material may be used loose as a polishing or finishing powder where its sharp cutting edges and corners will also function to advantage, and it may be used in bonded form to perform numerous other surfacing operations when fine finishes are desired.

Having thus described the invention, it is desired to claim:

1. The process of making an abrasive polishing material which comprises forming a fusion of alumina, atomizing the molten alumina of said fusion, quenching the atomized alumina to form separate shot-like alpha alumina bubbles, and, without further treatment which substantially changes the chemical composition of the bubbles, crushing the thus formed bubbles to form a mass of particles consisting predominantly of separate rhombohedral crystals and pyramidal fragments thereof.

,2. The process of making an abrasive polishing material which comprises forming a fusion of alumina, atomizing the molten alumina of said fusion, quenching the atomized alumina to form separate shot-like alpha aluminabubbles, and,

' without further treatment which substantially changes the chemical composition of the thus formed bubbles, crushing said bubbles by a gentle mulling action to a fineness of 600 mesh and finer so as to fracture the bubbles along the boundaries of the crystals making up the alumina bubbles.

3. The process of making an abrasive polishing material which comprises forming a fusion of alumina, .atomizing the molten alumina of said fusion, quenching the atomized alumina to form separate shot-like alpha alumina bubbles composed of fully developed crystals of alpha alumina, said crystals being substantially of microns and smaller, and, without further treatment which substantially changes the chemical composition of the bubbles, crushing the thus formed bubbles to a fineness of 600 mesh and finer to separate the individual' 'crystals.

4. An abrasive "surfacing material consisting predominantly of rhombohedral crystals of alpha alumina and sharp pyramidal fragments thereof,

' without further treatment which substantially changes the chemical composition of the bubbles, crushing said bubbles to form a mass of particles consisting predominantly of separate rhombohedral crystals and pyramidal fragments thereof.

5. An abrasive polishing material consisting substantially entirely of rhombohedral crystals of alpha alumina and sharp pyramidal fragments thereof, said polishing material being made by forming a fusion of alumina atomizing the molten alumina of said fusion, quenching the atomized alumina to form separate shot-like alpha alumina bubbles consisting of a large number of rhombohedral crystals of alumina which are not greater than 15 microns across along any of the major axes, and, without further treatment which substantially changes the chemical composition of the thus formed bubbles, crushing said bubbles by a gentle mulling action to a fineness of 600 mesh and finer so as to fracture the bubbles along the boundaries of the crystals making up the alumina bubbles.

6. An abrasive material having the pulverized fineness of 15 microns and under, the individual particles being substantially complete rhombohedral crystals of alpha alumina and pyramidal fragments thereof, said abrasive material being made by forming a fusion of alumina, atomizing the molten alumina of said fusion, quenching the atomized alumina to form separate shot-like alpha alumina bubbles consisting of a large number of rhombohedral crystals of alumina which are not greater than 15 microns across along any of the major axes, and, without further treatment which substantially changes the chemical composition of the thus formed bubbles, crushing said bubbles to form a mass of particles consisting predominantly of separate vrhombohedral crystals and pyramidal fragments thereof.

RAYMOND C. BENNER. FREDERICK A. UPPER. 

